Wetland restoration and hydrologic reconnection result in enhanced watershed nitrogen retention and removal

Abstract

Restoration of freshwater wetlands presents a potential water quality benefit via removal of nutrients, but complex and unresolved changes in nutrient cycling can occur following restoration. In this study, we evaluated N removal and release in a deltaic wetland under scenarios of hydrologic reconnection and sediment dredging, and we modeled potential downstream impacts of these restoration activities in a Bayesian framework. Denitrification, N₂O production, and anammox were measured via the isotope pairing technique in intact sediment cores. Anammox was not detected. Denitrification rates in the control scenario (78.2–87.2 μmol N m⁻² h⁻¹) were significantly higher than those under hydrologic reconnection and dredging scenarios (14.7–56.0 μmol N m⁻² h⁻¹). N₂O production rates were typical of wetland environments. Denitrification and N₂O production were stimulated shortly following simulated dredging, indicating a short-term response to sediment disturbance. Under hydrologic reconnection, NH₄⁺ availability was decreased, inhibiting coupled nitrification-denitrification. Despite a decrease in denitrification activity, the wetland has the capacity to remove up to 10% of stream NO₃⁻ following hydrologic reconnection. Restoration is predicted to fully mitigate NH₄⁺ delivery to a downstream eutrophic lake, yet permanent N removal may be reduced due to the decoupling of nitrification and denitrification.

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